Conventional megasonic cleaning tanks employ a fluid filled tank having substrate supports therein and a source of megasonic energy, (e.g., a transducer) coupled to the fluid for directing sonic energy through the fluid to the surfaces of a substrate or wafer supported therein. During megasonic cleaning, the transducer oscillates between a positive and a negative position at a megasonic rate so as to generate positive and negative pressures within the fluid, and thereby couple megasonic energy to the fluid. As the energy imparted to the fluid oscillates between positive and negative pressure, cavitation bubbles form in the fluid during negative pressure and collapse or shrink during positive pressure. This bubble oscillation and collapse gently cleans the surface of the wafer.
Particles cleaned from the wafer are carried upward via a laminar flow of fluid and flushed into overflow weirs coupled to the top of the cleaning tank. Thus, a supply of clean fluid is continually introduced to the cleaning tank from the bottom of the sidewalls thereof. Cleaning fluid distribution nozzles are positioned along the bottom of the sidewalls to supply various cleaning fluids through the same nozzles or through dedicated sets of nozzles.
Most conventional cleaning tanks position one or more transducers along the bottom of the cleaning tank. Acoustic waves from these transducers reflect from the surface of the cleaning fluid back into transducers, and interference results in reduced power density in the tank and reduced cleaning efficiency. Due to the limited area of the tank's bottom, the number, size, placement and shape of the transducers, fluid inlets, etc., often can not be freely selected for optimal performance.
In practice, megasonic cleaners experience a number of other limitations as well. For instance, transducers with higher power density assure better cleaning efficiency, but generate considerable heat during operation. Accordingly, cooling systems are often used to prevent degradation of adhesive material that attaches a transducer to materials that couple the transducer's acoustic power to the cleaning fluid and to prevent overheating of the power supply that could reduce the life of its electrical components. Such transducer cooling systems, however, undesirably increase the cost and complexity of a megasonic cleaning system.
An alternative approach has been to employ a cycled array of multiplexed transducers in which each transducer is on only 1/Nth of the cycle time, where N is the number of transducers per cleaning vessel. The reduction of duty cycle by a factor of N, which is usually 8 for 8 inch wafer batch processing vessels, reduces transducer temperatures and in some cases eliminates the need for transducer cooling systems. A major problem of this approach is the often unacceptable increase in processing time by a factor of N. The increase in processing time is particularly problematic for single wafer processing, where short processing time is an important requirement.
Another problem experienced by megasonic cleaners is the shadowing of the transducer's acoustic field by the wafer supports on which a wafer is positioned. Because a support must be positioned below the wafer to stabilize the wafer, the supports directly block energy transmitted from transducers positioned below the wafer.
Accordingly, a need exists for an improved sonic cleaning tank that provides high laminar fluid flow yet avoids the interference of incident and reflected waves, maintains short processing times without requiring transducer cooling arrangements, and minimizes shadowing by substrate supports.